OBJECTIVE: Research on the biological pathophysiology of autism has found some evidence that alterations in androgenic hormones may play a role in the pathophysiology of that disorder. We studied morning concentrations of serum testosterone in a very homogenic group of postpubertal youngsters with autism and a group of normal controls. METHODS: This study examines the serum testosterone concentration on 9 consecutive time points between 08.00 AM and 12.00 AM in 18 high- functioning male youngsters with autism (age 1218) and 22 healthy volunteers participated in this study. All subjects passed the onset of puberty (Tanner-stage IIIIV) and were of the Caucasian race. RESULTS: Repeated measures ANOVA revealed a significant time effect, with a decline in the testosterone concentration during the test and time X diagnosis interaction.The total testosterone concentration was significantly lower in the autism group compared to the group of normal controls. CONCLUSIONS: The significant decrease in serum testosterone concentration in male youngsters with autism suggest that the turnover of testosterone may take part in the pathophysiology of autism. Suggestions for further research are discussed.